Variable capacitors are used in many applications, such as matching networks and variable filters. They allow for the precise tuning, after assembly, of frequency and/or impedance in applications needing a dynamic system response, such as in plasma processes. The ability to dynamically change impedance and frequency response provides more flexibility for the applications variable capacitors are used in, and can compensate for variations from unit-to-unit. Some examples of variable capacitors are vacuum variable capacitors (VVCs) and electronically variable capacitors (EVCs).
In electronic circuits, matching networks are used to match the source impedance to the load impedance and vice versa. That is, the source, being of some impedance with a resistive part and a reactive part, will be terminated into the complex conjugate impedance, and the load impedance will be driven by the complex conjugate of its impedance. The complex conjugate is used to eliminate the reactive part of the impedance, leaving only the resistive part, and the resistive part is made equal. This is done so that maximum power transfer can be achieved at the load.
In plasma applications, the load impedance can vary depending on several factors, such as time, power level, pressure, gas flow, chemistry of the gasses, and whether the plasma has been struck. Accordingly, the matching network must be able to automatically vary itself to ensure that the maximum power transfer is achieved. This helps with repeatability in both the depositing and etching.
An inherent issue that arises with matching networks is that very high voltages can be generated internal to the network. Such voltages can negatively affect the internal components of the matching circuit. For example, the capacitors in the circuit can be damaged or destroyed from overvoltage, rendering the circuit ineffective and useless. Previous solutions to this issue have led to either increased component sizes or increases in the number of peripheral components.
VVCs typically use cylindrical or spiral plates that are inter-wound. The vacuum is used as the dielectric. To change capacitance, one set of electrodes is moved in or out of the other set, which changes the amount of overlap, which changes the capacitance. The most basic calculation for capacitance can be seen in the following equation, where C is capacitance, μ0 is the permittivity of free space, A is the overlapping area, and d is the distance between plates.
  C  =                    μ        0            ⁢      A        d  
As the overlapping area increases or decreases as the one electrode is moved in and out, the capacitance will increase or decrease respectively.
As voltages increase in variable capacitors, the potential for a voltage breakdown occurs. To overcome this issue, the distance between the electrodes is increased. This increase in distance will increase the size of the part. It will also reduce the capacitance proportionately to the distance. This requires the amount of windings to be increased so that area per distance is kept constant. Adding more windings again increases the size of the part and also adds to the cost.
EVCs use switches to add or remove fixed capacitors, such as an MLCC (multi-layer ceramic capacitor), in a circuit. The capacitor and switch are placed in series. This circuit is then placed in parallel with other capacitor/switch circuits. The parallel circuits allow the capacitors to be simply added or subtracted in the circuit, depending on how many switches are opened or closed. In the case where all the switches are open, the EVC will be at its lowest capacitance value. In the case where they are all closed, the EVC will be at its highest capacitance value.
Typically, the switch is the limiting factor in an EVC. The amount of voltage that the switch is capable of withstanding before dielectric breakdown occurs depends upon the type of switch (e.g., a PIN Diode, a Transistor, or a FET) and the switch's properties. Traditionally, to handle high voltages, more switches would be added in series to increase the breakdown voltage of the circuit. For example, if two switches are placed in series, the breakdown voltage is doubled. There are also other components that are needed to switch the device, and the circuit may become more complex with the added switches. All of these peripherals, extra switches and components, to the EVC's fixed capacitor, add to the costs and size of the EVC.
Thus, whether the variable capacitor in a matching network is a VVC or an EVC, there is need for a matching network that can more effectively handle high voltages generated in the network. There is further need for a solution that avoids or minimizes the need for increased component sizes (as typically required for a VVC) or increased numbers of peripheral components (as typically required with an EVC). There is further need for a solution that has a lower cost than previous methods of addressing high voltages in a matching network.